1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with the generation of frequency modulation tonal effects.
2. Description of the Prior Art
A musical tone generator which creates a waveshape that is time invariant will have a mechanical-like tone quality which easily fatigues a listener. This a negative reaction to a time invariant tone stimulus is a subjective emotion which can be attributed to a fatigue phenomena produced by an unchanging audible stimulation. Even a supposedly mechanical-like tone generator such as a wind blown organ pipe does not produce a constant time invariant audible sound. Turbulances in the air source produce noise-like tone modulations which are sufficient to provide amplitude and frequency tone modulations that combine to render the pipe tone less fatiguing to a listener.
A variety of prior art systems have been designed which introduce a frequency modulation to provide a time variant quality to a generated musical sound. In U.S. Pat. No. 3,794,748 entitled "Apparatus And Method For Frequency Modulation For Sampled Amplitude Signal Generating System" a system is described wherein a musical tone has a fundamental frequency which is frequency modulated. The disclosed system generates a musical tone by sequentially and repetitively addressing out a set of waveshape data points stored in a memory. The memory addresses are determined by a frequency number that is associated with an actuated keyboard switch. A frequency modulation is obtained by varying the assigned frequency number in a time variant manner.
In U.S. Pat. No. 4,175,464 entitled "Musical Tone Generator With Time Variant Overtones," there is described a tone generator in which waveform amplitude data is generated from a table of sinusoid values in an addressable memory by changing the addresses as a function of time in a periodic or sinusoidal manner. The effect is to produce a sequence of sinusoidal values from the table which correspond to a series of points on a frequency modulated carrier signal. By making the effective modulation frequency equal to the carrier frequency, the resulting frequency modulated signal corresponds to a carrier with side bands that correspond to the harmonics of the carrier.